In radios using the superheterodyne principle, all signal frequencies are typically converted to a constant lower frequency before detection. This constant frequency is called the Intermediate Frequency, or IF. Superheterodyne receivers “heterodyne” a frequency from a local oscillator (within the receiver) with the incoming signal. That is, they generate new frequencies by mixing two or more signals in a nonlinear device. A superheterodyne receiver converts any selected incoming frequency by heterodyne action to a common intermediate frequency where amplification and selectivity (filtering) are provided.
In the receiver of a mobile terminal the local oscillator signal is mixed in the receiver mixer with the received signal in order to generate the intermediate frequency signals. For an RF wireless application, a transmitter and receiver require a synthesizer to up convert and down convert modulated and received signals
In synthesizer design, various frequencies are combined to produce an intermediate frequency. The IF is the frequency to which all selected signals are converted for additional amplification, filtering and eventual direction. However, the combining of various frequencies often results in spurious signals. These spurs are unwanted signals produced by an active microwave component, usually at a frequency unrelated to the desired signal or its harmonics.
A frequency synthesizer generates any of a range of frequencies from a single fixed timebase or oscillator. Frequency synthesizers are used to generate the local oscillator signals required to perform the down conversion in the receiver and the up conversion in the transmitter. Frequency synthesizers generate multiple channels from a single master crystal oscillator, and can generate hundreds of frequencies.
Wireless communication systems typically require frequency synthesis in both the receive path and the transmit path.
Phase-locked loop (PLL) circuits including voltage controlled oscillators (VCOs) are often used in mobile terminal applications to produce the desired output frequency. Almost all modern synthesizers operate on the principle of the phase locked loop. Other systems exist, based on mixing, or on a combination of mixing and PLL designs. The frequency synthesizer compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies. The error signal is used to drive a voltage controlled oscillator which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This input is called the reference and is derived from a crystal oscillator. The key to the ability of a frequency synthesizer to generate multiple frequencies is that the divider is placed between the output and the feedback input.
A conventional directional coupler is shown in FIG. 1. A directional coupler couples part of the transmission power by a known amount out through another port, often by using two transmission lines set close enough together such that energy passing through one is coupled to the other. As shown in FIG. 1, a directional coupler generally has four ports: an input port (P1), a transmitted port (P2), where generally one half of the input signal is directed), a coupled port (P3), where one half of the input signal is directed, and an isolated port (P4) where no signal is directed. Often the isolated port is terminated with an internal or external matched load. It should be pointed out that since the directional coupler is a linear device any port can be the input. The term “main line” or “main path” refers to the section between ports 1 and 2. Moreover, the signals coming out from Ports 2 and 3 are 90 degrees out of phase with each other.
Reference can be made to, for example, commonly assigned U.S. Pat. No. 6,215,988, entitled “Dual-Band Architectures for Mobile Stations”, by Jorma Matero, for showing the conventional use of directional couplers for detecting the output TX power in part of a closed loop TX power control system.
Traditional superheterodyne transceivers have required the output of the VCO to down/up convert signals using mixers. This requirement means that the VCO signals must be divided into two paths to perform the up/down conversion.
Conventional methods for power division have included designs using either three resistors in a resistive divider configuration or a Wilkinson power divider type configuration. These conventional methods have their own advantages and disadvantages, but one common feature is that the power division to the RX and TX mixers is equal. In addition, these conventional techniques for VCO power division required a significant number of components, including an additional amplifier acting as a buffer in the up conversion path to provide high isolation from the up converter to the down converter, resulting in large current requirements and power consumption. Minimizing battery power consumption is a priority in wireless communications devices, as well as minimizing the physical size of the device.